Researchers have discovered an unexpected superconducting transition in extremely thin films of niobium diselenide (NbSe₂). Publishing in Nature Communications, they found that when these films become thinner than six atomic layers, superconductivity no longer spreads evenly throughout the material, but instead becomes confined to its surface.

This discovery challenges previous assumptions and could have important implications for understanding superconductivity and developing advanced quantum technologies.

Researchers at the Hebrew University of Jerusalem have made a surprising discovery about how superconductivity behaves in extremely thin materials. Superconductors are materials that allow electric current to flow without resistance, which makes them incredibly valuable for technology. Usually, the properties of superconductors change predictably when the materials become thinner; however, this study found something unexpected.

Superconductivity is a quantum phenomenon, observed in some materials, that entails the ability to conduct electricity with no resistance below a critical temperature. Over the past few years, physicists and material scientists have been trying to identify materials exhibiting this property (i.e., superconductors), while also gathering new insights about its underlying physical processes.

Superconductors can be broadly divided into two categories: conventional and unconventional superconductors. In conventional superconductors, electron pairs (i.e., Cooper pairs) form due to phonon-mediated interactions, resulting in a superconducting gap that follows an isotropic s-wave symmetry. On the other hand, in unconventional superconductors, this gap can present nodes (i.e., points at which the superconducting gap vanishes), producing a d-wave or multi-gap symmetry.

Researchers at the University of Tokyo recently carried out a study aimed at better understanding the unconventional superconductivity previously observed in a rare-earth intermetallic compound, called PrTi₂Al₂₀, which is known to arise from a multipolar-ordered state. Their findings, published in Nature Communications, suggest that there is a connection between quadrupolar interactions and superconductivity in this material.

If one side of a conducting or semiconducting material is heated while the other remains cool, charge carriers move from the hot side to the cold side, generating an electrical voltage known as thermopower.

Past studies have shown that the thermopower produced in clean two-dimensional (2D) electron systems (i.e., materials with few impurities in which electrons can only move in 2D), is directly proportional to the entropy (i.e., the degree of randomness) per charge carrier.

The link between thermopower and entropy could be leveraged to probe exotic quantum phases of matter. One of these phases is the fractional quantum Hall (FQH) effect, which is known to arise when electrons in these materials are subject to a strong perpendicular magnetic field at very low temperatures.